The invention concerns an optical fiber measuring device that can measure the variation in a parameter producing non-reciprocal interference in an SAGNAC ring interferometer.
The SAGNAC interferometer and the physical phenomena it involves are well understood. In an interferometer of this kind, the incident beam of light waves is split into two wave trains by a separating plate or some other splitting device. The two counter-propagative waves thus created travel in opposite directions along a closed optical path and on recombining, produce interference that is dependent on the difference in phase between the two waves at the time they recombine.
The closed optical path of SAGNAC interferometers was originally defined by mirrors. It is now known that this path can be formed by a multiturn coil of monomode optical fiber.
It is also known that some physical phenomenon can produce interference on the conter-propagative waves, particularly non-reciprocal dephasing, resulting in relative phase differences between the waves which modify their interference state at the time they recombine.
By measuring this relative phase difference, it is possible to quantify the phenomenon that caused it.
The main physical phenomenon likely to create this non-reciprocal interference is the SAGNAC effect, produced when the interferometer is rotated about an axis perpendicular to the plane of the closed optical path. The FARADAY effect or colinear magneto-optical effect is also known to produce non-reciprocal effects of this kind. Other effects also can produce a non-reciprocal phase difference under certain conditions.
By contrast, variations of many parameters representative of the environment and which often interfere with measurements, produce only reciprocal effects on the SAGNAC interferometer. They do not interfere with the relative phase difference between the counter-propagative waves, and do not thus influence the measurement of the parameter considered. Such is the case of slow variations in temperature, indexes, etc., which modify the optical path followed by the waves, but modify it reciprocally.
Numerous studies have been conducted with a view to improving the sensitivity and accuracy of the measurements that can be taken with such a measuring instrument. We can, for example, consult this subject in Chapter 9 "Fiber Optic Gyroscope" by Herve C. LEFEVRE in the work "OPTICAL FIBER SENSORS" Vol.2--ARTECH HOUSE--1989, and also the article entitled "Principle du gyrofibre, le gyrometre, des applications a haut dynamique" published in the Revue Scientifique de la Defense (1st quarter 1990) written by the same author.
Different signal processing methods are suggested. We should first of all note that the response supplied by the SAGNAC interferometer is in the form P=Po(1+cos .delta..PHI.), which means that the sensitivity of this signal in the vicinity of a phase difference .delta..PHI.=0 must be low. A suggestion was made to introduce a phase difference modulation .delta..PHI..sub.m, square in amplitude of plus or minus .pi./2 for example, which would move the working point and produce a periodic signal whose amplitude is a sinusoidal function of the measured parameter, and which can thus be used with greater sensitivity and stability. This phase difference is referred to as the biased phase difference .delta..PHI..sub.m.
This phase difference .delta..PHI..sub.m is produced by a phase modulator placed at one end of the interferometer's multiturn coil and controlled by a signal V.sub.m. This signal V.sub.m generates a phase shift .PHI.m on each wave and produces a phase difference between the counter-propagative waves: EQU .delta..PHI..sub.m (t)=.PHI..sub.m (t)-.PHI..sub.m (t-.tau.)
where t is time and .tau. the transit time of one of the waves in the coil.
It was then shown that it was possible to improve the accuracy of the measurement by using a zero method, also known as closed loop working. By this method, an additional so called counter-reaction phase difference .delta..PHI..sub.CR is applied, its purpose being to compensate for the phase difference .delta..PHI..sub.p produced by the measured parameter. The sum of these two phase differences, .delta..PHI..sub.CR and .delta..PHI..sub.p, is held at zero, which allows the interferometer to be operated at maximum sensitivity. The measurement is taken by using the signal required to produce the counter-reaction phase difference .delta..PHI..sub.CR. Measurement is thus stable and linear.
This phase difference .delta..PHI..sub.CR is produced by the phase modulator which is controlled by a signal V.sub.CR. This signal V.sub.CR generates a phase shift .PHI..sub.CR on each wave, resulting in a phase difference .delta..PHI..sub.CR between the counter-propagative waves: EQU .delta..PHI..sub.CR (t)=.PHI..sub.CR (t)-.PHI..sub.CR (t-.tau.)
The slaving required for this closed loop working can be obtained through a frequency shift. This shift can be generated directly using acoustic-optical modulators or simulated by applying serrodyne modulation to a phase modulator. Such serrodyne modulation is obtained by applying rising sawtooth phase modulation.
The interference likely to arise in a device of this kind through spectral instability of the source have been identified, and a solution to the problem was suggested in French patent FR-A-2 613 067. According to the technique described in this document, the modulation of the square of the phase difference, which allows the working point of the device to be moved, comprises asymmetric pulses, that is to say pulses whose amplitude alternates between maximum and null during different durations.
It is shown that due to the dispersion of the medium constituting the fiber of the interferometer, it is possible to extract two signals from the device, one being the function of the measured parameter and the other the function of the mean of the source. To achieve this, the light flux is spectrally split into two by a filter. Provided the two fluxes are produced by a filter that is stable, the two signals that are a function of the their respective mean wavelength can be used for slaving, i.e. to stabilize the source.
Also note that French patent FR-A-2 654 827 also suggested a method for processing the signal to ensure closed loop working of the interferometer. According to this technique, the biased phase difference .delta..PHI..sub.m is modulated on four successive values .PHI..sub.0, a.PHI..sub.0, -.PHI..sub.0, -a.PHI..sub.0. This particular modulation of the phase difference also provides a means of slaving the gain of the modulation command chain and offers different specific advantages.
To date, only very cumbrous means have been envisaged to overcome measurement uncertainty caused by spectral variations of the source in phase modulated systems. The possibility of using a reference interferometer has been suggested, and even that of introducing a miniaturized array spectrometer.